will adding a subwoofer improve overall audio

The Wilson benesch Torus will do all the above. It wouldn't improve the clarity of your speakers, but will the lower bass. Not a lot of buzz on it, though JV at TAS did an in depth review, pretty expensive at 10K. Since I am a dealer for WB I am sure my phone will be ringing with orders.

Boy in a small room wouldn't Gilman be better served to go with a better CD player?

I have Electra 936's and 2 JL Audio F113's and it is a HUGE improvement. I would have never bought these subs from reading posts or reviews. What I was reading however made me find some to demo. These add a great deal of depth and you will feel like you have different speakers once dialed in. They handle the higher frequencies with extreme control so don't be afraid to cross these over at 80hz. The sub also hides well at these higher crossover points. I think you would be impressed with the F113. The Velodyne DD18 gets my vote as well but the JL fit my area.

I have never been able to listen to any bookshelf speakers with woofers less than 5" on their own and always use a subwoofer to fill up the low registers. Yes, a subwoofer will make a huge difference to the overall sound especially if your speakers are small monitors or bookshelves with small drivers. Just make sure the subwoofer is of certain quality or standard and the setting-up is done correctly and you would be in for a treat.

I have tried running my mains (JM Lab 936) in full range with the JLA f113's. It did not work out as well. It seems that taking the strain off the amps opens things up in the upper mids and highs and these subs do a better job from 80 on down. More depth and very full.